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Recent improvements in intravascular magnetic resonance imaging techniques mandate an accurate method of monitoring the introduction of MR catheter probes into the vessel of interest. For this purpose, a novel imaging protocol and a display method have been designed. First, a roadmap 3D image data set with standard pulse sequences is obtained using an external imaging coil. Subsequently, using very narrow rectangular-FOV fast-spoiled gradient recalled (SPGR), a movie of the percutaneous placement procedure of an MR catheter probe is acquired at a rate of 7.3 frames/second. In this protocol, the probe is used to transmit RF pulses and receive MR signal. A computer program was written for image unwrapping and for displaying the unwrapped movie frames on the roadmap image. In an alternative protocol, the movie frames in two projection angles were acquired in an interleaved fashion. Frames were unwrapped and combined with a 3D roadmap and displayed on a Silicon Graphics workstation equipped with stereovision goggles. Using these methods, percutaneous catheter placement in a phantom and a dog was examined. In conclusion, a new visualization technique for MR catheter placement is proposed. Combining this technique with high resolution intravascular MRI techniques may result in a very useful diagnostic tool for the evaluation of atherosclerosis and other vessel diseases.

Multisite, passive microprobes have been developed to allow simultaneous recording of action potential activity from multiple neurons at different locations in the brain. The microprobes were fabricated using standard integrated circuit techniques. The probe is a planar structure that consists of gold electrodes sandwiched between two polyimide dielectric layers and bonded to a molybdenum structural support. Windows in the top dielectric layer expose the electrode sites and bonding pads. In two distinct versions of the probe four or six recordings sites, respectively, of approximately 25 microns 2 are arranged on a dagger-shaped structure which can penetrate the pia. The bonding pads and interconnect wires at the probe head are entirely encapsulated in a tubular fixture that is packed with silicone RTV and sealed with epoxy to protect the interconnections from contact with body fluids. The site impedances at 1 kHz are typically between 2 and 4 M omega. Probe lifetimes for continuous immersion in physiological saline solution, as measured by impedance, have exceeded 750 h. The failure mechanism is believed to be due to moisture and ion absorption in the top dielectric layer. In acute neurophysiological experiments using the four site probes, action potential activity was recorded from physiologically identified neurons in the dorsal column nuclei of anesthetized rat.